1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method used for ophthalmological consultations.
2. Description of the Related Art
Ophthalmic examinations are widely performed for the purpose of early diagnosis of lifestyle-related diseases and diseases that rank highly among causes of loss of eyesight. A scanning laser ophthalmoscope (SLO), which is an image processing apparatus that uses the principle of a confocal laser microscope, is an apparatus that performs Raster scanning of an eye fundus using a laser that is a measuring beam and obtains a planar image at a high resolution and a high speed based on the intensity of the return light. The apparatus that captures this planar image will be referred to as an SLO apparatus, and the planar image will be referred to as an SLO image below.
In recent years, it has been possible to obtain a retinal SLO image with an improved horizontal resolution by increasing the diameter of the measuring beam in the SLO apparatus. However, there has been a problem in acquiring a retinal SLO image in that increasing the diameter of the measuring beam is accompanied by a decrease in the S/N ratio and the resolution of the SLO image due to aberrations in the eye of the examination subject.
In order to resolve the above-mentioned problem, an adaptive optics SLO apparatus has been developed that has an adaptive optics system that measures aberrations in the eye of the examination subject in real-time using a wavefront sensor and corrects aberrations of a measuring beam or its return light that occur in the examination subject eye using a wavefront compensation device, thereby enabling the acquisition of an SLO image with a high horizontal resolution.
This SLO image having a high horizontal resolution can be acquired as a moving image, and in order to observe blood flow dynamics, for example, in a non-invasive manner, retinal blood vessels are extracted from the frames of the moving image, and the movement speed and the like of blood cells in capillaries are subsequently measured. Also, in order to evaluate the relationship between the photoreceptor cells and the visual function using the SLO image, photoreceptor cells P are detected, and subsequently the density distribution and the alignment of the photoreceptor cells P are measured. FIG. 6B shows an example of an SLO image with a high horizontal resolution. The photoreceptor cells P, a low luminance region Q that corresponds to the position of a capillary, and a high-luminance region W that corresponds to the position of a leukocyte can be observed.
In the case of observing the photoreceptor cells P, measuring the distribution of photoreceptor cells P, or the like using the above-described SLO image, the focus position is set near the retinal outer layer (B5 in FIG. 6A between innermost layer B1 to pigment layer B6) and an SLO image such as FIG. 6B is captured. On the other hand, there are retinal blood vessels and bifurcated capillaries in the retinal inner layers (B2 to B4 in FIG. 6A). 45% of the blood that exists in blood vessels is composed of blood cell components, and of those blood cell components, about 96% are erythrocytes and about 3% are leukocytes. An erythrocyte has a diameter of about 8 μm, and a neutrophil, which is the most common type of leukocyte, is about 12 to 15 μm in size.
As shown in FIG. 6C, if a leukocyte W is moving in a capillary in flow direction FD, small erythrocytes R flowing in the rear cannot pass the large leukocyte in the front, and therefore erythrocytes accumulate and an aggregation (hereinafter referred to as an “erythrocyte aggregate”) forms behind the leukocyte. The size of this kind of erythrocyte aggregate Dti is the smallest immediately subsequent to a vascular bifurcation (FIG. 6C), and it increases gradually as it nears the next vascular bifurcation (FIGS. 6D and 6E). Note that this aggregation occurs physiologically, and if the leukocyte is no longer in front of the erythrocyte aggregate, the erythrocytes will separate and move individually once again. If the focus position is set to the photoreceptor cells and an SLO image having a high horizontal resolution is acquired, the erythrocyte aggregate DTi will be rendered as a dark tail behind the high-luminance leukocyte region W, as shown in FIG. 6F.
On the other hand, with a diabetic patient for example, erythrocytes will aggregate abnormally and form erythrocyte aggregates DTi regardless of whether or not a leukocyte W is present, as shown in FIG. 6G. In this latter case, erythrocyte aggregates DTi will be present at various positions in the capillary, including the position behind a leukocyte W. Since the erythrocytes are regularly aggregated, the length of the erythrocyte aggregate DTi behind the leukocyte will hardly change when moving between capillary bifurcations. Accordingly, by calculating the change in the size of the erythrocyte aggregate DTi behind the leukocyte based on an SLO moving image with a high horizontal resolution, blood fluidity (the extent to which blood flows smoothly) can be measured in a non-invasive manner.
However, in the case of measuring blood fluidity using multiple SLO images that were captured in the same site in the eye area, there has been a problem in that it is cumbersome to find out the change over time. Also, there has been a problem in that the reliability of measured values is low since statistical values for measured values are measured/calculated without giving consideration to the timing of the heartbeat. Therefore, a technique for    (i) quantifying the change over time in measured values for the blood fluidity of the eye area in a simple manner, and    (ii) improving the reliability of the measured values for the blood fluidity of the eye area is needed.
Non-Patent Document 1 discloses a conventional technique for non-invasively measuring blood fluidity in which a spatiotemporal image is generated using a capillary branch region in an adaptive optics SLO image and the degree of change in the length of a blood cell aggregate in the spatiotemporal image is measured.
[Non-Patent Document 1] Uji, Akihito, “Observation of dark tail in diabetic retinopathy using adaptive optics scanning laser ophthalmoscope”, Proceedings of the 66th Annual Congress of Japan Clinical Ophthalmology, p. 27 (2012).
However, the technique above does not give consideration to simply and accurately obtaining a measurement position that corresponds to a measurement position of an image that was captured in the past in order to easily measure and display the change over time in measured values in images captured at different timings, for example. Also, in the technique above, it is not disclosed that the measured values are more accurately calculated with consideration given to the timing of the heartbeat, for example.